Closed loop analog gyro rate sensor

ABSTRACT

The present invention provides an apparatus and method for measuring the angular rotation of a moving body. The apparatus comprises an upper sensor layer, a lower handle layer substantially parallel to the sensor layer, at least one dither frame formed of the upper sensor layer, the frame having a dither axis disposed substantially parallel to the upper sensor layer and the lower handle layer. The apparatus further comprises a first accelerometer formed of the upper sensor layer and having a first force sensing axis perpendicular to the dither axis for producing a first output signal indicative of the acceleration of the moving body along the first force sensing axis, the first accelerometer having a proof mass and at least one flexure connecting the proof mass to the dither frame such that the proof mass can be electrically rotated perpendicular to the dither axis. The apparatus also comprises a second accelerometer formed of the upper sensor layer and having a second force sensing axis perpendicular to the dither axis for producing a second output signal indicative of the acceleration of the moving body along the second force sensing axis, the second accelerometer having a proof mass and at least one flexure connecting the proof mass to the dither frame such that the proof mass can be electrically rotated perpendicular to the dither axis. The dither frame and proof masses have electrodes on an insulating layer for operating the first and second accelerometers and the upper sensor layer has a rate axis perpendicular to each of the first and second force sensing axes and the dither axis, whereby the first and second output signals have a Coriolis component indicative of the angular rotation of the moving body about the rate axis.

REFERENCES TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 10/136,525, filedApr. 29, 2002.

BACKGROUND

1. Technical Field of the Invention

The present invention relates to an apparatus and methods fordetermining the acceleration and rate of angular rotation of a movingbody, and in particular, one which is adapted to be formed, for examplethrough micromachining, from a silicon substrate.

2. Description of the Prior Art

A variety of methods and systems are known for determining theacceleration and rate of angular rotation of a moving body. Such methodsand systems have found their way in a diverse range of applications, oneof which is military. However, the use of tactical grade inertiameasuring units has been limited by their cost to high-priced systemssuch as military aircraft, missiles, and other special markets. The costof inertia measuring units is dominated by the expensive discretegyroscopes and discrete accelerometers and attendant electronics used todrive and convert these signals for use in computer systems.

Other problems with inertial measuring units are high power consumptionand large package size. The problems of high power consumption and largepackage size further limit applications to larger equipment boxes inequipment racks. For example, a hockey puck sized tactical gradenavigator is not known in the prior art.

Still other problems with the prior art, discussed below in more detail,include a limitation in rate bias accuracy caused by modulation of theaccelerometer due to coupling from the dither motion which causes phaseangle sensitivity of the rate data. A further limitation in rate biasaccuracy is caused by modulation of the accelerometer due to coupling ofexternal vibration components coupling into the rate data.

Exemplary rate and acceleration sensors, components of such sensors, andmethods of forming the same are described in the following patents allof which are assigned to the assignee of this disclosure: U.S. Pat. Nos.5,005,413; 5,168,756; 5,319,976; 5,331,242; 5,331,854; 5,341,682;5,367,217; 5,456,110; 5,456,111; 5,557,046; 5,627,314; 6,079,271;6,098,462; and 6,276,203.

By way of background, the rate of rotation of a moving body about anaxis may be determined by mounting an accelerometer on a frame anddithering it, with the accelerometer's sensitive axis and the directionof motion of the frame both normal to the rate axis about which rotationis to be measured. For example, consider a set of orthogonal axes X, Yand Z oriented with respect to the moving body. Periodic movement of theaccelerometer along the Y axis of the moving body with its sensitiveaxis aligned with the Z axis results in the accelerometer experiencing aCoriolis acceleration directed along the Z axis as the moving bodyrotates about the X axis. A Coriolis acceleration is that perpendicularacceleration developed while the body is moving in a straight line,while the frame on which it is mounted rotates. This Coriolisacceleration acting on the accelerometer is proportional to the velocityof the moving sensor body along the Y axis and its angular rate ofrotation about the X axis. An output signal from the accelerometer thusincludes a DC or slowly changing component or force signal Frepresenting the linear acceleration of the body along the Z axis, and aperiodic component or rotational signal Ω representing the Coriolisacceleration resulting from rotation of the body about the X axis.

The amplitude of that Coriolis component can be produced by vibratingthe accelerometer, causing it to dither back and forth along a lineperpendicular to the input axis of the accelerometer. Then, if the frameon which the accelerometer is mounted is rotating, the Coriolisacceleration component of the accelerometer's output signal will beincreased proportional to the dither velocity. If the dither amplitudeand frequency are held constant, then the Coriolis acceleration isproportional to the rotation rate of the frame.

The linear acceleration component and the rotational componentrepresenting the Coriolis acceleration may be readily separated by usingtwo accelerometers mounted in back-to-back relationship to each otherand processing their output signals using summed difference techniques.In U.S. Pat. No. 4,510,802, assigned to the assignee of the presentinvention, two accelerometers are mounted upon a parallelogram withtheir input axes pointing in opposite directions. An electromagneticD'Arsonval coil is mounted on one side of the parallelogram structureand is energized with a periodically varying current to vibrate theaccelerometers back and forth in a direction substantially normal totheir sensitive or input axes. The coil causes the parallelogramstructure to vibrate, dithering the accelerometers back and forth. Bytaking the difference between the two accelerometer outputs, the linearcomponents of acceleration are summed. By taking the sum of the twooutputs, the linear components cancel and only the Coriolis orrotational components remain.

U.S. Pat. No. 4,590,801, commonly assigned to the assignee of thepresent invention, describes the processing of the output signals of twoaccelerometers mounted for periodic, dithering motion to obtain therotational rate signal Q and the force or acceleration signal Frepresenting the change in velocity, i.e. acceleration, of the movingbody along the Z axis.

U.S. Pat. No. 4,510,802, commonly assigned to the assignee of thepresent invention, describes a control pulse generator, which generatesand applies a sinusoidal signal of a frequency ω to the D'Arsonval coilto vibrate the parallelogram structure and thus the first and secondaccelerometer structures mounted thereon, with a dithering motion of thesame frequency ω. The accelerometer output signals are applied to aprocessing circuit, which sums the accelerometer output signals toreinforce the linear components indicative of acceleration. The linearcomponents are integrated over the time period T of the frequency ωcorresponding to the dither frequency to provide the force signal F,which represents the change in velocity, i.e., acceleration, along the Zaxis. The accelerometer output signals are also summed, whereby theirlinear components cancel and their Coriolis components are reinforced toprovide a signal indicative of frame rotation. That difference signal ismultiplied by a zero mean periodic function sgnc (ωt). The resultingsignal is integrated over a period T of the frequency (ω by a sample andhold circuit to provide the signal Ω representing the rate of rotationof the frame.

The D'Arsonval coil is driven by a sinusoidal signal of the samefrequency ω which corresponded to the period T in which the linearacceleration and Coriolis component signals were integrated. Inparticular, the pulse generator applies a series of pulses at thefrequency ω to a sine wave generator, which produces the substantiallysinusoidal voltage signal to be applied to the D'Arsonval coil. A pairof pick-off coils produce a feedback signal indicative of the motionimparted to the accelerometers. That feedback signal is summed with theinput sinusoidal voltage by a summing junction, whose output is appliedto a high gain amplifier. The output of that amplifier, in turn, isapplied to the D'Arsonval type drive coil. The torque output of theD'Arsonval coil interacts with the dynamics of the parallelogramstructure to produce the vibrating or dither motion. In accordance witha well known in the art servo theory, the gain of the amplifier is sethigh so that the voltage applied to the summing junction and thefeedback voltage are forced to be substantially equal and the motion ofthe mechanism will substantially follow the drive voltage applied to thesumming junction.

U.S. Pat. No. 4,881,408 describes the use of vibrating beam forcetransducers in accelerometers. In U.S. Pat. No. 4,372,173, the forcetransducer takes the form of a double-ended tuning fork fabricated fromcrystalline quartz. The transducer comprises a pair of side-by-sidebeams which are connected to common mounting structures at their ends.Electrodes are deposited on the beams and a drive circuit applies aperiodic voltage signal to the electrodes, causing the beams to vibratetoward and away from one another, 180 degrees out of phase. In effect,the drive circuit and beams form an oscillator with the beams playingthe role of a frequency controlled crystal, i.e., the mechanicalresonance of the beams controls the oscillation frequency. The vibratingbeams are made of crystalline quartz, which has piezoelectricproperties. Application of periodic drive voltages to such beams causethem to vibrate toward and away from one another, 180 degrees out ofphase. When the beams are subjected to accelerating forces, thefrequency of the mechanical resonance of the beams changes, whichresults in a corresponding change in the frequency of the drive signal.When subjected to acceleration forces that cause the beams to be placedin tension, the resonance frequency of the beams and thus the frequencyof the drive signal increases. Conversely, if the beams are placed in acompression by the acceleration forces, the resonance frequency of thebeams and the frequency of the drive signal is decreased.

Above referenced U.S. Pat. No. 5,005,413 describes accelerometersutilizing vibrating force transducers that require materials with lowinternal damping to achieve high Q values that result in low drivepower, low self-heating and insensitivity to electronic componentvariations. Transducer materials for high-accuracy instruments alsorequire extreme mechanical stability over extended cycles at high stresslevels. Crystalline silicon possesses high Ω values, and with the adventof low cost, micromachined mechanical structures fabricated fromcrystalline silicon, it is practical and desirable to create vibratingbeams from a silicon substrate. Commonly assigned U.S. Pat. No.4,912,990 describes a vibrating beam structure fabricated fromcrystalline silicon and including an electric circuit for applying adrive signal or current along a current path that extends in a firstdirection along a first beam and in a second, opposite direction along asecond beam parallel to the first. A magnetic field is generated thatintersects substantially perpendicular the conductive path, whereby thefirst and second beams are caused to vibrate towards and away from oneanother, 180 degrees out of phase.

Digital techniques employ stable, high frequency crystal clocks tomeasure a frequency change as an indication of acceleration forcesapplied to such vibrating beam accelerometers. To ensure preciseintegration or cosine demodulation, a crystal clock is used to setprecisely the frequency of the dither drive signal. Outputs from twoaccelerometers are fed into counters to be compared to a reference clocksignal produced by the crystal clock. A microprocessor reads thecounters and processes the data to provide a force signal F and arotational signal Ω. The main advantage of digital processing is theability to demodulate with extreme precision. The short term stabilityof the reference crystal clock allows the half cycle time basis to beprecisely equal. Thus, a constant input to the cosine demodulator isdivided up into equal, positive half cycle and negative half cyclevalues, whose sum is exactly zero.

In an illustrative embodiment, the two accelerometer signals are countedin their respective counters over a 100 Hz period (corresponding to 100Hz of the dither frequency ω) and are sampled at a 400 Hz data ratecorresponding to each quarter cycle of the dither motion. The twoaccumulated counts are subtracted to form the force signal F. Since thecounters act as an integrator, the acceleration signal is changeddirectly to a velocity signal. Taking the difference of the accelerationsignals tends to reject all Coriolis signals as does the counterintegration and locked period data sampling.

The Coriolis signals are detected by a cosine demodulation. The cosinedemodulated signals from the first and second accelerometers are summedto produce the Δθ signal. Again, the counters integrate the rate data toproduce an angle change. The sum also eliminates any linear accelerationand the demodulation cancels any bias source including bias operatingfrequency and accelerometer bias. The accelerometer temperature is usedin a polynomial model to provide compensation for all the coefficientsused to convert the frequency counts into output units. Thus, the scalefactor, bias and misalignment of the sensor axes are corrected over theentire temperature range.

The demodulation of the frequency sample is straightforward once thedata is gathered each quarter cycle. The cosine demodulation is simplythe difference between the appropriate half cycles. The linearacceleration is the sum of all samples.

Various issues with the use of vibrating beam force transducers inaccelerometers include the need to operate the device in a substantialvacuum such that the beams can vibrate at their natural frequencywithout loss of energy from viscous damping. Also, the vibrating beamsof the first and second accelerometers are formed in first and secondlayers of epitaxial material formed on opposing sides of the siliconsubstrate so that the force sensing axis of each accelerometer isdirected opposite to the direction of the other. In other words, thevibrating beams must be on opposing sides of the substrate so that onewill be in compression and the other in tension when subjected to anapplied acceleration force. The high doping levels in the epitaxiallayer required to form the vibrating beams make the material inherentlyunstable. Thus, the output of the vibrating beams tends to degrade overtime and with exposure to thermal environments. The nature of vibratingbeam transducers causes accelerometer design and analysis to berelatively complex as compared to that of simpler force rebalanceaccelerometers and their larger size reduces the quantity ofaccelerometers which can be fabricated in a single wafer of siliconsubstrate so that vibrating beam accelerometers are inherently moreexpensive to produce than miniature force rebalance accelerometers.

Miniature silicon force-rebalance accelerometers in an integratedcircuit form are small and inexpensive and generally have a largedynamic range and are operable in high vibration environments over awide temperature range. Miniature silicon force-rebalance accelerometershaving a silicon proof mass suspended between a pair of electrode layersand responsive to differential capacitive coupling between the electrodelayers and the proof mass for opposing acceleration forces applied tothe proof mass are described in U.S. Pat. No. 4,336,718. The miniaturesilicon force-rebalance accelerometer of the prior art includes a proofmass and two flexures integrally formed from a silicon substrate. Theflexure preferably defines a bend line along the mid-plane of the proofmass which is intended to minimize vibration rectification. The siliconsubstrate including the proof mass is anodically bonded between upperand lower glass substrates having upper and lower metal, for example,gold, electrodes deposited thereon. The upper and lower substrates arepreferably formed identically. Symmetry between opposing surfaces of theproof mass and between opposing the electrodes deposited on the upperand lower glass substrates surfaces minimizes bias and maximizes dynamicrange and linearity.

The state of the art in micromachined rate and acceleration sensors isrepresented by U.S. Pat. No. 5,341,682 which is commonly assigned to theassignee of the present invention and incorporated herein by reference.Rate and acceleration sensors, as disclosed in U.S. Pat. No. 5,341,682,are comprised of two accelerometers aligned in a single plane such thatthe input or sensitive axes of the two accelerometers are parallel andthe output or hinge axes of the two accelerometers are parallel. The twoaccelerometers are vibrated or dithered at a predetermined frequencyalong a dither axis parallel to the hinge axes. The two accelerometerstend to vibrate at slightly different frequencies due to slight massmismatch. Even if driven by a drive signal of common frequency, theaccelerometer motions tend to be out of phase with each other. A link isconnected to each of the two accelerometers whereby motion imparted toone accelerometer results in like, but opposite motion imparted to theother accelerometer. Thus, the dithering motion imparted to oneaccelerometer is ideally of the exact same frequency and precisely 180degrees out of phase with that applied to the other accelerometer.

The link provides an interconnect between the two accelerometers whichis stiff in the dither axis such that the motion imparted to oneaccelerometer is effectively transmitted to the other accelerometer andboth accelerometers ideally dither at the same frequency and precisely180 degrees out of phase. The link is pivotally fixed to the frame by apivot flexure. The link is further connected to each of the twoaccelerometers by flexures. The link is typically formed in a complexasymmetric shape. The complexity of the link is driven by practicalconsiderations involved in adapting the link to accommodate both, thepivot flexure and the two link-to-accelerometer flexures. The link'scomplex asymmetric shape provides adequate clearance between the linkand the frame for the pivot flexure. The link's shape also providesadequate clearance between the link and each accelerometer to providethe precise flexure length to ensure that the flexures exhibit apredetermined mix of simple arc bending and “S-bend” motion and toensure that any motion imparted to one accelerometer by the flexures isimparted to the other accelerometer as a sinusoidal function withoutintroducing a higher order harmonic into the translation motion.

Although the device described in above referenced U.S. Pat. No.5,341,682 functions for the purposes intended, its exact behavior isdifficult to predict and/or model analytically. For example, the complexshape of prior links results in spring rates which are asymmetrical anda shape which is difficult to solve analytically. Additionally,constructing the shape previously taught results in flexures whosethicknesses and hence vibration properties are difficult to control.Therefore, later patents, for example, U.S. Pat. No. 6,098,462 and U.S.Pat. No. 6,079,271, provide links having simple geometric shapes formedsymmetrically about the pivot point. The behavior of these simplersymmetric links is more easily predicted and/or modeled analytically.For example, these simpler symmetric links result in spring rates whichare symmetrical and easier to solve analytically using conventionalmethods. Additionally, constructing the simpler symmetric shape resultsin flexures whose thicknesses and hence vibration properties are moreeasily controlled.

U.S. Pat. No. 6,098,462, which is commonly assigned to the assignee ofthe present invention, provides a linkage between accelerometers in amicromachined rate and acceleration sensor which is relatively simple tosolve analytically and results in flexures whose thicknesses arerelatively insensitive to process variations. For example, the linkshape can be solved using classical mechanical equations. In addition,the U.S. Pat. No. 6,098,462 provides a simple symmetrically shaped link,which is relatively insensitive to process variations, having sufficientmechanical stiffness to effectively transmit motion imparted to oneaccelerometer to the other accelerometer such that both accelerometersdither at the same frequency and precisely 180 degrees out of phase. Thesimple symmetrical link provides adequate clearance between the link andeach accelerometer for flexures having a length which ensures that theflexures exhibit a predetermined mix of simple arc bending and “S-bend”motion and which ensures that any motion imparted to one accelerometerby the flexures is imparted to the other accelerometer as a sinusoidalfunction without introducing a higher order harmonic into thetranslation motion. The link having a columnar shape in the dithercross-axis has a reduced sensitivity to cross-axis vibration.

As described in U.S. Pat. No. 5,341,682, the accelerometers aresuspended from a dither or mounting frame by a pair of flexures or“dither legs” which vibrate upon application of a dithering force totranslate the accelerometers in a predominantly linear relationship witheach other. However, true orthogonality is not achieved between thedither motion and the Coriolis acceleration sensing direction in thenormal manufacturing process. State of the art micromachined vibratingCoriolis rate and acceleration sensors, as represented by U.S. Pat. 25No. 5,341,682 and others of the above incorporated patents, experiencequadrature motion due to the manifold sources of mechanical imperfectionresulting from the tolerances inherent in manufacturing processes. Thisaxis misalignment in conjunction with a phase shift causes a rate biaserror which limits performance.

One method and apparatus for overcoming the errors introduced byquadrature motion is described in U.S. Pat. No. 5,886,259, assigned tothe assignee of the present invention, steers the accelerometer inputaxes to be orthogonal using capacitive attraction between the sensingmass and stationary members of the sensor frame. However, suchcapacitive steering requires very small gaps, on the order of microns,between the sensing mass and stationary members to generate sufficientapplied force. Given the very small gaps necessary, actualimplementation of this axis alignment feature is difficult in a normalmanufacturing setting using conventional processing methods.

The prior art as discussed above, however, has proven unsatisfactory.Generally, the Coriolis rate sensors have the problem of difficultfabrication, lengthy analysis and hard to implement axis-alignmentcapability. The result of such shortcomings is less than optimal yields,low quantity of sensors per wafer, higher design costs and limited ratebias performance.

Accordingly, it is clear that there exists a need for a cost effectivemethod and apparatus for determining the acceleration and rate ofangular rotation of a moving body which overcomes the above mentionedproblems.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a system and method foraccurate and low cost measurement of rate and linear acceleration in ahigh volume design. The method includes automatically nulling out themodulation using closed loop electronics without the need for mechanicalsensor trimming or extraordinary processing tolerances.

In accordance with one embodiment of the present invention, a system isprovided using a self contained steering mechanism for axis-alignmentcorrection as well as measuring both rate and linear acceleration. Thesystem comprises an analog closed loop alignment servo, a pair ofcapacitive force rebalanced accelerometers, a unique magnetically drivendither mechanization using a compact link and self-contained dampingplates for the accelerometers.

In accordance with another embodiment of the present invention, a methodof axis-alignment is provided. The method comprises the steps offabricating into the accelerometer a rotational feature to allow theforce rebalance to operate a robust axis-alignment correction. Thepresent invention provides an apparatus and method for measuring theangular rotation of a moving body. The apparatus comprises an uppersensor layer, a lower handle layer substantially parallel to the sensorlayer, at least one dither frame formed of the upper sensor layer, theframe having a dither axis disposed substantially parallel to the uppersensor layer and the lower handle layer. The apparatus further comprisesa first accelerometer formed of the upper sensor layer and having afirst force sensing axis perpendicular to the dither axis for producinga first output signal indicative of the acceleration of the moving bodyalong the first force sensing axis, the first accelerometer having aproof mass and at least one flexure connecting the proof mass to thedither frame such that the proof mass can be electrically rotatedperpendicular to the dither axis. The apparatus also comprises a secondaccelerometer formed of the upper sensor layer and having a second forcesensing axis perpendicular to the dither axis for producing a secondoutput signal indicative of the acceleration of the moving body alongthe second force sensing axis, the second accelerometer having a proofmass and at least one flexure connecting the proof mass to the ditherframe such that the proof mass can be electrically rotated perpendicularto the dither axis. The dither frame and proof masses have electrodes onan insulating layer for operating the first and second accelerometersand the upper sensor layer has a rate axis perpendicular to each of thefirst and second force sensing axes and the dither axis, whereby thefirst and second output signals have a Coriolis component indicative ofthe angular rotation of the moving body about the rate axis.

It is to be understood that both the foregoing summary and the followingdetailed description of the present invention are exemplary and areintended to provide a description of, and not limit, the presentinvention.

The present invention will now be described in greater detail, withfrequent reference being made to the drawings identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram of a sensor in accordance with one embodiment of thepresent invention;

FIG. 2 is a diagram of a sensor stack-up in accordance with oneembodiment of the present invention;

FIG. 3 is a diagram of an accelerometer force rebalance circuit inaccordance with one embodiment of the present invention;

FIG. 4 is a diagram illustrating an axis alignment method in accordancewith one embodiment of the present invention;

FIG. 5 is a diagram of an inertial measurement unit in accordance withone embodiment of the present invention;

FIG. 6 is a block diagram of electronic circuitry in accordance with oneembodiment of the present invention; and

FIG. 7 is a block diagram of a central processing unit in accordancewith one embodiment of the present invention.

FIG. 8 is a schematic diagram of a sensor in accordance with anembodiment of the present invention.

FIG. 9 is another view of the sensor of FIG. 8.

FIG. 10 is a circuit diagram in accordance with an embodiment of thepresent invention.

FIG. 11 is a circuit diagram in accordance with another embodiment ofthe present invention.

FIG. 12 is a side cross-sectional view of the embodiment of FIG. 11.

FIG. 13 is a schematic side cross-sectional view of a manufacturing stepin the preparation of the embodiment of FIG. 11.

FIG. 14 is a side cross-sectional view of another embodiment of thepresent invention.

FIG. 15 is a side cross-sectional view of another embodiment of thepresent invention.

FIG. 16 is a top view of the embodiment of FIG. 15.

FIG. 17 is a circuit diagram of the embodiment of FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is presented to enable any person of ordinaryskill in the art to make and practice the present invention.Modifications to the preferred embodiment will be readily apparent tothose of ordinary skill in the art, and the disclosure set forth hereinmay be applied to other embodiments and applications without departingfrom the spirit and scope of the present invention and the appendedclaims. Thus, the present invention is not intended to be limited to theembodiments described, but is to be accorded the broadest scopeconsistent with the claims appended hereto and the disclosure set forthherein.

The present invention relates to an analog silicon version of L-3Communications' existing vibrating beam, bulk etched, rate andacceleration sensor, which is a part of L-3 Communications' existingμIMU (micro Inertial Measurement Unit). Inertial Measurement units(IMUs) are critical to the operation of inertial navigation and guidancesystems. Such systems are used on ships, aircraft, spacecraft, etc. Atypical IMU utilizes a cluster of three accelerometers and three gyrosmounted to a structure which is shock isolated. The three accelerometersare used to measure linear acceleration while the gyros are used tomeasure angular rate.

The vibrating beam multi-sensor is known in the industry as μSCIRAS™(Micromachined Silicon Coriolis Inertial Rate and Acceleration Sensor).The analog sensor version of μSCIRAS™ (referred to as the “closed-loopanalog” or CLA) in accordance with the present invention is a magneticdrive, capacitive pickoff rate and acceleration sensor with anaxis-alignment servo. Preferably, the sensor has less than 1°/hr biasinstability. While the key features of the closed loop gyro according tothe present invention are implemented in a thinner version of μSCIRAS, asimpler deep reactive ion etch (DRIE) process is preferably employed.

The analog CLA design in accordance with the present invention usessimilar etching processes as the current bulk-etched sensor which iswell known in the art. However, using DRIE to define an 80 micron thick,two-dimensional projection, a much simpler process is employed tofabricate the gyro and accelerometer according to one embodiment of thepresent invention. Since the entire moving portion of the sensor is auniform 80 microns thick, local processing variations are nulled outusing electrostatic forces in a servo loop driven by an error detector.

In accordance with the preferred embodiment of the present invention, a1°/Hr rate bias stability, less than 0.5 mg accelerometer biasstability, and better than 0.1°/√Hr rate random walk are achieved. Theanalog gyro approach of the present invention has the advantage of beingable to filter the signals to achieve a low noise for the angle randomwalk. By incorporating a closed loop alignment servo, the rate bias canbe achieved without manual trimming.

The 1°/hr rate sensor would fit such programs as WCMD (Wind CorrectedMunition Dispenser) or other GPS-aided inertial navigation programs. Theideal sensor for such a program would be a silicon based, axis-alignmentcorrected rate sensor with very low inherent power. Preferably, amagnetic drive, capacitive pickoff rate sensor with an axis alignmentservo, would be less than 1°/hr bias over temperature in accordance withthe present invention. An active axis-alignment would essentiallyeliminate rate bias as the dominant error source. The same sensor wouldexhibit less than 0.05 deg/√hr.

Basic Concept

The basic rate and acceleration sensor for one axis consists of a singlesilicon mechanism mounted on an electronic hybrid substrate with thenecessary analog drive electronics. According to the embodiment of thepresent invention, several of the key tradeoffs have been made withrespect to general configuration. The size of the sensor in accordancewith the present invention is approximately 1/5th of the current wellknown in the art bulk silicon μSCIRAS . This provides a productioncapability of about 250 sensors per 200 mm wafer, which is an increaseof about 5 times the production employed by the prior art. The wafersare the standard thickness of 680 μm, with very light doping levels.

FIG. 1 illustrates one embodiment of the upper sensor layer arrangementcontaining a rate and acceleration sensor 10 according to the presentinvention. The size of the diced wafer including a top cover and thesensor is about 0.27″×0.27″ or about 6.8 mm squared compared with theexisting prior art mechanism of 0.7″×0.55″.

The upper sensor layer is grown by known methods on one surface ofunitary, silicon substrate. First and second accelerometers aremicromachined from the upper sensor layer and are disposed inside-by-side relation such that their input axes 15A and 15B aredisposed in parallel but opposite directions. In FIG. 1, input axis 15Aof the first accelerometer is disposed to the top of the page, whereasinput axis 15B of the second accelerometer is disposed to the bottom ofthe page. Further, input axes 15A and 15B are disposed perpendicular toa dither or vibration axis 20 and to a rate axis 25. As is well known inthe art, the first and second accelerometers will respond to linearacceleration along their input axes 15A and 15B, respectively, and torotation of the upper sensor layer about its rate axis 25.

The upper sensor layer includes a mounting frame. Each of accelerometersis rotationally suspended relative to the upper sensor layer by a pairof hinge flexures. Hinge flexures are well known in the art. Forexample, U.S. Pat. No. 6,257,057 teaches that the length of each ofhinge flexures is selected to provide a spring rate relative to the massof 25 accelerometers, that will cause hinge flexures to flex in apredominately simple bending motion combined with a component of“S-bend” motion when subjected to an acceleration force applied alonginput axis 15. Hinge flexures do double duty and act to transmit thedither motion to the accelerometers in the plane of the upper sensorlayer. Each of dither flexures are preferably formed of one or morenarrow beams 30, upon which conductive paths are deposited. Configuringdither flexures to have multiple narrow beams or “legs” decreases thein-plane stiffness thereby minimizing the force that drives the dithermotion. The spring rate of narrow beams or legs 30 of dither flexures isproportional to W³ /L³ in the plane of the upper sensor layer, where Wis the width of legs and L is the length of legs. The length L and widthW of hinge legs 30 are set such that when rotational motion is appliedin the plane of the upper sensor layer, legs 30 resist motion. Inresponse to a Coriolis acceleration, the proof mass flexures will flexin an “S” configuration. Such “S-bend” flexures permit accelerometers totranslate with predominantly linear motion in the plane of the uppersensor layer, i.e., the plate surfaces of proof masses 35A, 35B ofaccelerometers remain substantially parallel to the surfaces of theupper sensor layer and to each other as they are translated along theirrespective input axes 15A and 15B.

Magnet and flux path with case return provide a magnetic path fordirecting the flux emanating from the magnet through the upper sensorlayer. As will be explained, the configuration and disposition ofaccelerometers within upper sensor layer permits a simple,straightforward magnetic flux path to effect the operation of thedithering motion of accelerometers. Upon application of a periodic drivesignal or current to conductive path via external connectors,interaction with a magnetic field emanating from magnet substantiallyperpendicular to the surface of the upper sensor layer subjectsaccelerometers to a periodic dithering motion along common dither axis20.

A link 40 is connected to each frame opposite hinge flexures to insurethat the dithering motion imparted to one of the accelerometers will beof the same frequency and in phase with that applied to the other ofaccelerometers. Consequently, link 40 mechanically interconnects firstand second accelerometers so that any motion, including dithering motionand extraneous motions applied to one of first and secondaccelerometers, will also be applied in precisely equal and oppositefashion to the other one of first and second accelerometers. In thisfashion, the outputs of accelerometers may be processed simply by wellknown sum and difference techniques to provide a force signal F and therotational signal Ω, as well as to cancel out erroneous signals. Withoutlink 40 there between, accelerometers would tend to vibrate at slightlydifferent frequencies due to slight mass mismatch. Even if driven by adrive signal of common frequency, the motions of the accelerometerswould tend to be out of phase with each other. Link 40 is connected byflexures to the moving side of the first frame opposite to ditherflexures, which mount the first accelerometer to the mounting frame.Link 40 is similarly connected by a second dither flexure to the movingside of the second accelerometer opposite to dither flexures, whichmount the second accelerometer to the mounting frame. Link 40 ispreferably axes-symmetric about a pivot point and, according to onepreferred embodiment, is symmetrically shaped. Alternatively, link 40 isany of a complex asymmetric ‘U’ shape as described in U.S. Pat. No.5,241,861 or a simple ‘U’ shape symmetric about pivot point as describedin each of U.S. Pat. Nos. 6,098,462 and 6,079,271. Link 40 is supportedby two pivot flexures. Pivot flexures are in turn mounted along a centeraxis of the upper sensor layer by support members, which are in turnaffixed to an accelerometer frame 45.

Those of ordinary skill in the art will recognize that proof masses 35Aand 35B are subject to motion in the plane of the upper sensor layeralong respective input axes 15 in response to linearly appliedacceleration forces. Furthermore, those of ordinary skill in the artwill recognize that the combination of proof mass 35 with flexures forma mass-spring system that has a natural resonance frequency alongrespective input axes 15.

Preferably, the mass-spring system formed by the combination of link 40with pivot flexures and support members has a resonant frequencyessentially matched to that of the mass-spring system formed by proofmass 40 and hinge flexures such that the motion of the link/flexuresystem does not couple into that of the proof mass/flexure system.

The first and second accelerometers are analog electrostaticforce-rebalance accelerometers as taught, for example, in each of U.S.Pat. Nos. 5,350,189 and 5,205,171. Analog force-rebalance accelerometershave distinct advantages over the vibrating beam accelerometers used inthe rate and acceleration sensors of the prior art as exemplified byU.S. Pat. No. 5,241,861. One advantage of the analog force-rebalanceaccelerometers in the rate and acceleration sensor of the presentinvention is that noise can be filtered with a quadratic low-pass filterat the signal source whereby random walk can be confined to a relativelynarrow range on the order of 0.05 deg/hour over a relatively widebandwidth, for example, a 150 Hz bandwidth. Another advantage is thataxis alignment using an analog sensor can be implemented by therelatively simple means of summing an off-set voltage to the capacitormotor plates. By feeding back a sine demodulated rate channel output, ahigh bandwidth servo loop can continuously null the major bias errorsource and compensate for thermal stresses, mechanical packing stresses,initial processing errors and wafer flatness errors. Also, analogforce-rebalance accelerometers are typically operated in one atmospherepressure which reduces sealing integrity requirements to limits readilyrealizable using conventional sealing methods, such as welding. Themagnetic dither drive described herein provides sufficient force todrive the mechanism in one atmosphere ambient pressure.

The dithered accelerometers are driven magnetically. As discussed above,the two dithered accelerometers (accels) are connected by a link 40 andpreferably operate at a dither frequency of about 8 kHz. The twoproof-masses are an “H” shape of approximately 1.2 mm high by 2.5 mmhigh. On the proof mass and the dither frame are electrodes for theforce rebalanced accelerometer. These are on an oxide layer and are theactive electrodes.

Upon application of the dithering motion, accelerometers move back andforth in a substantially parallel relationship to each other due to the“S-bend” flexing of the dither flexures. The bending motion resemblestwo smooth curves, the first terminating at the center point in onedirection and the second curve with an opposite curvature meeting thefirst curve at the center point. “S-bend” four pairs of dither flexuresensure that accelerometers move in an essentially linear motion, wherebythe horizontal and vertical edges of proof masses 35A and 35B remainprecisely parallel with the inner horizontal and vertical peripheraledges of accelerometer frame 45. The back-to-back orientation ofaccelerometers ensures that the summed outputs of the accelerometersprovide an accurate indication of linear acceleration. In addition,extraneous movements acting on the accelerometers will, at least to afirst order of measure, tend cancel or dampen each other, wherebyextraneous signals do not appear in the summed accelerometer outputs. Inan analogous fashion when the difference of outputs of theaccelerometers is taken, the canceling characteristics of these curvesensure that second order nonlinearities in the resultant angularrotation signal will also average.

In accordance with the preferred embodiment of the present invention,the sensor is self contained on one silicon wafer. Therefore, the sensorthickness can be defined by an SOI (Silicon On Insulator) wafer toplayer, for example. The upper sensor layer in which accelerometers areformed is positioned parallel to a lower handle layer as illustrated inFIG. 2. The two wafers are bonded at an oxide layer. The upper sensorlayer can be thinned to the desired thickness. The buried oxide acts asan etch stop for a wet etch such as TMAH (tetramethyl ammoniumhydroxide) or DRIE. The oxide can be chemically removed or otherwiseetched away. Thus, the sensor is easily fabricated using RIE down to theoxide. The handle wafer can be removed by potassium hydroxide (KOH) etchdown to the oxide, then the exposed oxide can be removed by chemicaletch.

Due to being a self contained sensor, the electrical connections areonly on one layer, the upper sensor layer. The device is defined by theinherent accuracy of one mask. For example, there is no need to alignthe upper and/or lower cover wafers for the device to function. Inaddition, the accelerometer input axes 15 are defined in-plane of thesilicon, which means the gaps can be precisely set by the mask, ratherthan etching and bonding of cover plates, thereby resulting in a betterperformance of the sensor. The accelerometer is a translational massinstead of a pendulous mass, which means that the accelerometer axisdoes not change direction with g-input, which is beneficial to theperformance. Moreover, the orthogonality of the accelerometer axis withrespect to dither is defined in the mask orthogonality which is not afunction of RIE process orthogonality which is not as good or uniformover the entire wafer. This reduces the single largest error source in aCoriolis sensor—quadrature acceleration coupling into the rate axis.

Accordingly, the sensor sandwich consists of the substrate with uppercover plate as shown in FIG. 2. The top cover plate 205 has windowsetched over the electrical pad connections. After the sensor substratedetails are cut with the DRIE, an electrical contact is made to thesubstrate by a metal contact. This substrate contact applies a voltageto the entire structure. The sandwich is put together using a glass fritscreened on in a continuous square on the top cover plate. Preferably, a12 μm gap is established on the covers using a short KOH or DRIE etch todefine the frit thickness.

During operation, each of the accelerometers balance the externallyapplied acceleration forces by applying electrical restoring forces toproof masses 35 through upper and lower electrodes. Both AC and DCsignals are applied to upper and lower electrodes. Capacitances formedbetween upper electrode and a first side of proof masses 35 and betweenlower electrode and a second side of proof masses 35 are coupled to aforce rebalance circuit which is discussed in detail below. Theforce-rebalance circuit drives an electrostatic restoring force based onthe difference in capacitance thereby balancing the applied accelerationforces to restore each of proof masses 35 to a neutral position betweenopposing lower and upper capacitor plates in frame 45.

Force-Rebalance Circuit

During operation, an applied acceleration causes relative motion betweenproof mass 35 and upper and lower electrodes as proof masses 35 attemptto translate about proof mass flexures. An imbalance in capacitancesbetween proof masses 35 and upper and lower electrodes results. Thesensor circuitry balances the capacitance imbalance by applying anelectromotive force (EMF) to each of upper and lower restoringelectrodes to move each proof mass 35 to a neutral position betweenupper and lower electrodes and hold it there. For example, as proof mass35 moves and approaches one of upper electrodes and simultaneouslyrecedes from the lower electrode, an increased capacitive pickup of theAC signal causes a compensation circuit to apply a decreased DC signalvoltage to the approaching electrode and to increase the signal voltageapplied to the receding electrode, whereby an electrostatic force isapplied to each proof mass 35 to resist the force of acceleration andrestore each proof mass 35 to a neutral position.

The polarity of upper and lower electrodes are arranged to form the aaccelerometer having a positive output in response to a positivelyapplied acceleration force and a second accelerometer having a negativeoutput in response to a positively applied acceleration force, wherebythe sensed acceleration is determined by taking the difference betweenoutputs of first and second accelerometers, and the Coriolis rate isdetermined by demodulating the sum of the outputs of first and secondaccelerometers.

FIG. 3 illustrates one example of an accelerometer force-rebalancecircuit. Other useful circuits are known to those of ordinary skill inthe pertinent art, for example, each of U.S. Pat. Nos. 4,336,718 andU.S. Pat. No. 5,205,171 teach useful accelerometer forcerebalancecircuits.

The force rebalance accelerometer circuit is shown in FIG. 3. Itconsists of a differential capacitance detector, compensation amplifier,offset bias amplifiers and output summing amplifier. In theaccelerometer force-rebalance circuit 400, the signal of upperelectrodes of sensor assembly is output at 405A, the signal of lowerelectrodes is output at 405B, and the signal of proof mass 35, which isa center tap in this embodiment, is output at 405C. Output signals 405are input to synchronous demodulator 410 of known design. The output ofsynchronous demodulator 410 feeds a compensation integrator circuit 415.The output of compensation integrator circuit 415 is inverted byinverting operational amplifiers 420 a and 420 b and summed separatelyinto each offset and each electrode. Accelerometer force-rebalancecircuit 400 also provides for the alignment voltage to be summed intothe feed-back signal at node 425 and subtracted from the output at node430, whereby the summing in of the alignment signal is transparent, i.e.not visible, in the analog accelerometer output signal 435.

The proof mass capacitance of each plate of the pick-off capacitors ofthe accelerometer is preferably about 3 pf with a nominal 5 μm gap. So acapacitive pick-off circuit is used as the front-end circuit. A simplegain, amplifier, which doubles as a compensation integrator 415(integrates out to about 4 kHz, then is just a fixed gain) is the onlycompensation circuit needed because the gas damping is set-up for Q=8 at8 kHz. The electrodes are preferably biased off at +6 VDC.

According to another embodiment of the present invention, a single 5Vsupply circuit can be designed. The output of the pick-off feeds thecompensation integrator, which is summed in separately to each offsetand electrode. The polarity of the electrodes is set-up to form a plusand minus accel so that acceleration is the difference of the accels andrate is the demodulated sum of the accels. The quadratic filter on theoutput of the rate channel acts as a bandwidth limiter, noise reducerand ripple eliminator. The ripple will be down 90 dB at the 5 output.

The channel block diagram is shown in FIG. 3. A difference amplifierforms the linear acceleration output. A low-pass filter is used to setthe bandwidth and filter out any remaining dither induced rate signals.A summing amplifier is used for the rate AC signal path that can beamplified to increase sensitivity. This signal is then demodulated toform a rate proportional voltage. Similarly, a low-pass filter sets thebandwidth and reduces the ripple to be compatible with a 16 bit A/Dconverter at 80 Hz. The dither loop is a closed loop oscillator thatruns at the natural frequency of the silicon mechanism. The velocity issensed and used to drive the accelerometers perpendicular to their inputaxes. The output of the loop is used as an input to a 90 degree phaseshifter to drive the rate channel demodulator. Since the accelerometerbandwidth is set exactly to the dither frequency, there is a 90° phaseshift of the rate data. The phase shifter matches this as well asfiltering the dither crossing noise.

To do the active axis-alignment a sine demodulator can be used andfed-back as an offset to the servo loops. This will couple in a smallamount of the turn-around acceleration from the dither to cancel anysine modulation from any source, such as thermal, mechanical packagestress, processing errors and wafer mask errors.

Referring to FIG. 4, in accordance with the preferred embodiment of thepresent invention, in order to achieve the end goal of 1°/hr biasstability, it is preferable to servo the orthogonality of theaccelerometer sense axes to the dither axes. This is achieved throughcapacitive offset of the accelerometer input axis. The four capacitorplate sections allow the proof mass “H” to be slightly rotated about themiddle by applying an offset voltage to diagonal ends of the “H” asillustrated in FIG. 4. This will require a circuit design to allow botha force rebalance top to bottom of “H” offset and axis-alignment ordiagonal corner offset be applied at the same time to null the inputaxes. Thus, quadrature signal rejection can be done at the source toeliminate a large bias error. Secondly, two blocks are dithered inopposite directions to reduce base reactions. They are linked to allowstable operation in the presence of vibration and to dynamically removelinear acceleration from the rate channel by summing and differencing ofthe two readings. This approach greatly simplifies the design andprocessing aspects of the prior art three-dimensional approach.

Spring Cancellation Feature

In accordance with another embodiment of the present invention, a methodof improving the accelerometer bias performance is provided through acancellation of the mechanical spring by adding a bias voltage acrossthe motor capacitor gaps. The motor capacitor consists of a stator platelocated on the dither frame and a proof mass plate located on themovable proof mass which is electrically coupled to the substrate uppersensor layer. A large negative voltage is applied to the substrate, butthe motor plate stays at a relatively small positive value. The resultis a large static field across the gap causing an attractive force. Asthe smaller of the two gaps to the capacitor fingers are mirror imagesabout the proof mass flexures there is a balanced bias force, the tophalf pulling the proof mass up and the bottom half pulling the proofmass down. The net result is a cancellation of static forces except ascommanded to the motors by differentially changing the top motor voltagewith respect to the bottom motor voltage. However, as the motor force isproportional to the voltage squared, the bias voltage now enters intothe commanded force as (V_(bias))×(V_(command)), or a larger productthan just the V_(command) squared. Thus, not only does the bias cancelthe mechanical flexure spring, but it also magnifies the motor forceallowing a greater g-range to be within servo control.

For example, the proof mass natural frequency is 5500 Hz due to the massand flexure suspension. A bias voltage of 54V can be added by using a−47 V supply connected to the sensor substrate and a +6V electricaloffset to the motor terminals on the insulated gold metallization. Thisresults in an attractive force that has a negative spring rate. In otherwords, the attraction increases as the gap grows smaller, therebyeffectively canceling the mechanical spring. The proof mass naturalfrequency drops from the 5500 Hz to less than 400 Hz when the bias isapplied. This reduces the bias errors induced by anything that changesthe capacitor null. For example, a 1 mV null error would have produced a60 mg error without the bias would now be less than 5 mg's. The same istrue for mechanically produced position changes.

Sensor Assembly

Referring to FIG. 5, the IMU includes a baseplate including one or moremounting flanges. Cubic return path is securely mounted to baseplate andsupports a nonmagnetic sensor mounting structure, upon which one or moreanalog control boards are mounted. Three completed sensors can bemounted on the adjacent faces of a cubic return path. Magnetic fluxgenerators are mounted on the adjacent faces of a cubic return pathunder each sensor in a non-critical location to the sensor's middle 0.2″by 0.1″ area. Magnetic flux generators are, for example, permanentmagnets or electromagnets, or suitable another magnetic flux generatorof a type known to those of skill in the pertinent art. Each sensor willhave a surrounding electronics hybrid to run the drive frequency and thetwo force rebalance accelerometers. Each analog control board supports asensor assembly including an upper sensor layer having first and secondaccelerometers formed therein such that their respective input axes aredisposed in parallel but opposite directions. Each accelerometer iselectrically interconnected to its respective control board which isin-turn interconnected by, for example, flex strip to analog processingcircuitry. The analog processing circuitry performs all of theaforementioned signal processing to allow for outputting angular rateand linear acceleration data to a central processing unit (CPU) viaanother flex strip (not shown).

A top cover seals in a prescribed atmosphere and protects the sensitivemechanical and electrical components from contamination and damage. Thesealed in atmosphere is preferably a standard dry nitrogen at onestandard atmosphere pressure.

Magnetic Dither Drive

Dither drive circuits are known to those of skill in the pertinent art.For example, U.S. Pat. Nos. 5,241,861 and 4,590,801 illustrate suchcircuits. The invention uses a commonly known dither drive circuit toapply a sinusoidal voltage across the effective portions of conductivedither path. Conductive dither path forms first effective portiondeposited on beams or dither legs 30 of dither flexures for imparting avibrating motion to the first accelerometer and second effective portionsimilarly deposited on beams or dither legs 30, of dither flexures forimparting a vibrating motion to the second accelerometer. A magneticfield is generated perpendicular to the surfaces of the upper sensorlayer and is directed through accelerometers and the effective portionsdisposed thereon. Effective portions are connected in series betweenexternal connectors. A single drive voltage applied to effectiveportions through external connectors generates a current in externalconnectors. The flux generated by the magnet interacts with the currentflowing through the effective portions to create a dither drive force Fwhich moves accelerometers in a substantially rectilinear, vibratingmovement back and forth along dither axes 20.

Accelerometers vibrate or dither at their resonant, natural frequency f₀determined by the mechanical characteristics of the sensor assembly,including the spring rates of proof mass flexures and the mass ofaccelerometers. The dither drive signal outputted by the dither drivecircuit is of a frequency corresponding to the frequency f₀ of dithervibration and, as explained above, is used in the further processing ofthe accelerometer outputs to demodulate those signals to provide a forcesignal F and a rotational signal Ω.

The conductive dither pick-off path is deposited on the top surface ofthe upper sensor layer, whereby a pick-off signal is generated whichrepresents the dither motion imparted to accelerometers by the ditherdrive current passing through conductive dither path. As accelerometersare vibrated, effective pick-off portions move through the magneticfield created by the unitary magnet, a current is induced therein andthe resultant voltage is fed back to the dither drive signal.

The configuration of accelerometers within the upper sensor layer andflux path generated by magnet, case return and cubic return pathdevelops a considerable force in excess of the damping losses tomaintain the dither motions of accelerometers. Those of ordinary skillin the art understand that a minimum turn-around acceleration is neededto cause each of accelerometers to stop going in one direction and toaccelerate in the opposite, whereby the dithering motion may occur. Theacceleration force F tending to maintain the dithering motion ofaccelerometers is set out by the following equation:F=L·i×B   Equation (1)where i is the current passing through the conductive path making up theeffective; portions, L is the effective length of that portion of theconductive path within the magnetic flux passing through accelerometers,i.e., the length of the effective portions, and B is the magnitude ofthe flux.

The drive acceleration a may be calculated by the following equation:$\begin{matrix}{\alpha = \frac{{D( {2f} )}^{2}}{K}} & {{Equation}\quad(2)}\end{matrix}$where D is the displacement, f is the dither frequency and K is aconversion factor.

The voltage induced in the pick-off portions, ε, is provided in thefollowing equation:ε=νx·BL   Equation (3)where ν is the amplitude of the velocity output signal ofaccelerometers; B is the strength of the magnetic field crossing theeffective portions; L is the effective length of the conductor withinthe magnetic flux field.

The accuracy with which rate and acceleration sensor 10 may be made, thesymmetry of accelerometers and their suspension by the flexures, and theinterconnection of link 40 to impose equal and opposite motions onaccelerometers, have an accumulative effect to greatly simplify theprocessing of the accelerometer output signals, essentially reducing itto a cosine demodulation step. This can be done every half cycle as wasshown in the prior art. Basically, the outputs of accelerometers aresubtracted from each other to provide the linear acceleration signal andto average both signals while inverting every other sample to demodulatefor the cosines to produce a rate of rotation signal ω. Neither analignment servo nor a phase servo is needed for such processing thusincreasing the band width of the rotational acceleration signal Ω to be0.5 kHz in one illustrative embodiment of this invention.

Rate and acceleration sensor 10 has a sensitivity to rotationalacceleration imposed about its rate axis, i.e., the moment of each ofaccelerometers about rate axis, which acceleration sensitivity willintroduce an undesired noise component in the subsequent demodulationprocessing of the-accelerometer output signals. As taught in U.S. Pat.No. 5,241,861, that noise component can be effectively eliminated bydifferentiating the rotation rate signal ω and scaling it. In effect,the demodulated outputs of accelerometers are a measure of its rotationrate signal ω, which can be differentiated to obtain an indication ofthe angular acceleration of each accelerometer. Since the dimensionsand, in particular, the distance between rate axis and each of center ofgravity is known to a high degree of precision, that equivalent radiusof rotation is multiplied by a measured angular acceleration force toobtain an accurate indication thereof of the linear acceleration causedby the angular acceleration. The calculated acceleration moment issubtracted from the accelerometer outputs to reduce or substantiallyeliminate such acceleration sensitivity.

FIG. 6 shows the channel block diagram which includes a differenceamplifier 505 to form the linear acceleration output from accelerometers510A and 510B. A low pass filter 515 is used to filter out any remainingdither induced rate signals in the linear acceleration output. Theacceleration output signals are demodulated in cosine demodulator 525Aand 525B to form rate proportional voltages. A summing amplifier 520 isused to amplify the cosine demodulated signals to increase sensitivity.Similarly, a low-pass filter 530 sets the bandwidth and reduces theripple such that the signal is compatible with an analog-to-digital(A/D) converter, for example, a 16-bit analog-to-digital converter at a80 Hz data rate.

The dither drive circuit 535 may take the form of that circuit shown inU.S. Pat. Nos. 5,241,861 and 4,590,801 which illustrate such circuits oranother suitable dither drive circuits as are known to those of skill inthe pertinent art. Dither drive circuit 535 is a closed loop oscillatorthat runs at the natural frequency of sensor assembly 540. The velocityis sensed by dither pick-off circuit and used in dither drive circuit todrive accelerometers perpendicular to acceleration input axes 15. Theoutput of the dither drive loop is used as an input to a 90 degree phaseshifter 545 to drive the rate channel demodulator. Active axis alignmentis performed by sine demodulators 550A and 550B which feed back offsetsto the servo loops of accelerometers 510. A small amount of theturn-around acceleration from dither drive circuit 535 is used sinedemodulators 550A and 550B to cancel any sine modulation from sourcessuch as thermal stress, mechanical packaging stress, processing errorsand wafer mask imperfections.

CPU Description

FIG. 7 illustrates the central processing unit (CPU) 600 in which thesix analog sensor outputs from the three sensor assemblies mounted onorthogonal faces of cubic return path are converted to a digital formatusing V/F converters. A bank of V/F converters 605 continuously read therate and acceleration voltages and output from each of the three sensorassemblies and output a digital frequency that is a conversion to a 24bit resolution data word using a digital ASIC counter 630. Data rates of1440 degrees/second can be resolved to a 1 degree/hour rate using a24-bit resolution data word. Central processing unit 600 is optionally agenerally available low cost micro-controller type (ASIC) capable ofhandling the data reads, correcting for bias and scale factor overtemperature using computer models, and correcting for axis misalignment,whereby corrected data is provided to a host computer for theperformance of navigation functions. The inertial measuring unit (IMU)processor would require an external circuit card or could be integratedin functionally with the host processor. Central processing unit 600further preferably includes flash memory 610, random access memory (RAM)615, a serial universal asynchronous receiver-transmitter (UART) 620 anda voltage regulator 625, for example, 3.3V regulator as shown in FIG. 6.

As explained in detail in above incorporated U.S. Pat. No. 4,786,861, Δνis provided by the following equation:Δν_(i) =A[(N1_(i) −N2_(i))+FT+B(N1_(i) +N2_(i))]  Equation (4)

where ν_(i) is the “ith” sample of the velocity signal, A and F arescale factors, N1_(i) is the count derived from a first counter over a 1k Hz (1 m sec) period for the “ith” sample, N2_(i) is the count obtainedfrom a second counter for the “ith” sample, T is the time period and Bis the bias correction term. As well known in the art, Δθ_(i) isprovided by the following equation:Δθ_(i) =a(cos N1_(i)+cos N2_(i))+b(cos N1_(i)−cos N2_(i))   Equation (5)where a is a scale factor and b is a bias/correction term, andcos (N1_(i))=N1_(i) −N1(_(i−1)), over each 8 kHz period or   Equation(6)cos (NI_(i))=(−1)^(i) N1_(i), at 8 kHz rate   Equation (7)

Angular acceleration a is equal to the linear acceleration as derivedfrom the output of either of accelerometers, divided by the equivalentradius of rotation, r_(eq) in accordance with the following equation:α=A _(linear) /r _(eq)   Equation (8)

In turn, angular acceleration a is a function of the measured rotationrate ω in accordance with the following equation:α=dω/dt   Equation (9)In turn, the rotation rate may be expressed as follows:ω=dθ/dt   Equation (10)

Since the derivative of the rotation rate ω is equal to acceleration α,acceleration may be expressed by the following equation: $\begin{matrix}{\alpha = {\frac{\omega_{i} - \omega_{({i - 1})}}{\Delta\quad t} = \frac{\frac{{\Delta\theta}_{i}}{\Delta\quad t} - \frac{\Delta\quad\theta_{({i - 1})}}{\Delta\quad t}}{\Delta\quad t}}} & {{Equation}\quad(11)}\end{matrix}$Thus, correction for linear acceleration A_(linear) is provided by thefollowing equation: $\begin{matrix}{A_{linear\_ correction} = {{\alpha\quad r_{eq}} = {{r_{eq}\frac{\omega_{i} - \omega_{({i - 1})}}{\Delta\quad t}r_{eq}} = \frac{\frac{\Delta\quad\theta_{i}}{\Delta\quad t} - \frac{\Delta\quad\theta_{({i - 1})}}{\Delta\quad t}}{\Delta\quad t}}}} & {{Equation}\quad(12)}\end{matrix}$In turn, the microprocessor is programmed in a conventional fashion tosubtract values of A_(linear) correction from the accelerometer outputsf1 and f2 to correct for angular acceleration.

The above discussion describes a highly advantageous CLA that combinesthe gyro and accelerometer functions on a single, small-size chip. In afurther development, the structure is improved to reduce the noise inthe output signal from the output pickoff capacitors.

More particularly, each of the two pickoff capacitors used foroutputting a measurement signal from the accelerometers is an aircapacitor of approximately 1.5 pf or slightly less. However, when themetal traces for carrying the measurement signals are attached, eachtrace itself creates a parasitic capacitance of about 30 pf with respectto the substrate. In the embodiments of the invention discussed above,the substrate is connected to a large negative voltage source at, forexample, −27, −37, −40 or −45 volts, which will be generally referred toherein at the motor common voltage Vmc and which is shared by all thefunctions of the CLA sensor. The motors use this voltage source incommon, but the voltage source itself generates a considerable amount ofnoise and the 30 pf capacitances couple this voltage noise and any motornoise from the substrate to the output traces, creating significantnoise in the signals.

FIG. 8 schematically shows the substrate S1 connected to the motorcommon voltage Vmc (here −37V) to carry this voltage to the pads P1-P4leading to the four motors in the two pairs of accelerometers. FIG. 9shows the motor teeth MT flanking the pickoff teeth PT for all fourmotors M1, M2, M3, M4, and it is the need to carry Vmc to all fourmotors through the substrate SI underneath the pickoff teeth PT thatleads to the noise transferred by the undesirable capacitive coupling.

It would therefore be highly desirable to reduce or eliminate the largecapacitive coupling between the noisy voltage source and the outputtraces.

One way of addressing this problem is to reduce the amount of capacitivecoupling between the negatively charged substrate and each output trace.It is basic electrical engineering that putting a small capacitance inseries with a large capacitance results in a small overall capacitance.Therefore, a first embodiment of this aspect of the invention comprisescreating a small capacitance in series with each 30 pf capacitance bybuilding a respective diode pair, called a dual diode structure,connected between the substrate and the respective pickoff capacitor.

This structure is shown in FIG. 10, wherein a differential pair ofpickoff air capacitors Cair 702, 704 provides a differential outputsignal. The substrate 706 is indicated by the resistance structure 708.Also indicated are the two 30 pf capacitances Csub 710, 712 respectivelybetween the pickoff capacitors 702, 704 and the substrate 706. In orderto reduce this capacitive coupling, one advantageous embodiment of thepresent invention provides an upper diode pair 714, 716 connectedbetween the upper side of the upper pickoff capacitor 702 and thesubstrate 706 and a lower diode pair 718, 720 connected between thelower side of the lower pickoff capacitor 704 and the substrate 706.Each diode pair is effectively a p-n-p structure with no lead connectedto the center n layer, providing a small capacitance relative to thesubstrate 706.

The capacitance of this diode structure itself is about 0.2 pf, whilethe lead on the top surface of the CLA has a capacitance of about 0.5pf. Accordingly, there is still the possibility that there will be noisecoupling to the front end electronics. Additionally, the diode structurehas a leakage current of about 3 nA at room temperature that doublesevery 10° C. to 192 nA at 85° C. This leads to an offset in the pickoffnull that rises exponentially with temperature, which in turn gives riseto an accelerometer bias thermal term that is generally hard to modelout.

To address these issues, a further development of the present inventionprovides another embodiment in which a second diode pair is added toisolate the pickoff capacitors, with the second diode pair tied to a−2.25V reference voltage for the front-end amplifier.

This structure is shown in FIG. 11, wherein a second upper diode pair722, 724 is added between the first upper diode pair 714, 716 and thesubstrate 800, and a second lower diode pair 726, 728 is connectedbetween the first lower diode pair 718,720 and the substrate 800. Thecenter n layers of these second diode pair p-n-p structures are eachtied to the −2.25V reference. As a result, the capacitance of each leadwire C_(lead) is between the tie at −2.25V and the respective pickoffcapacitor, not between the substrate at Vmc and the respective pickoffcapacitor.

FIG. 12 is a side cross-sectional view of this embodiment. Here, theP-type silicon substrate 800 is selectively covered with a bulk siliconoxide layer 802 in defined areas 802 a-e as will be described. From leftto right in FIG. 12, a via 803 between areas 802 a and 802 b permits themotor common voltage Vmc on wire 804 from the LTCC board to be connecteddown to the substrate 800. Metallization traces 808 for connections tothe pickoff capacitors pass over area 802 b and are isolated therebyfrom Vmc. Another via 806 between areas 802 b and 802 c permits Vmc tocome up from the substrate 800 to a trace 812 to the motors.

Further along the substrate surface, after the indicated “break,” a via814 between area 802 d and 802 e receives a lead from a pad 816 for the−2.25V reference voltage from ASIC. A pickoff pad 818 is spaced from pad816 on area 802 e and isolated thereby from Vmc. The trace 820 to therespective pickoff capacitor extends from the pickoff pad 818.

Underlying the via 814 with pad 816 thereon is an N-type doped bulksilicon guard layer 822 provided in the substrate 800. Guard layer 822extends under area 802 e to a region 823 forming a part of a diodestructure 824 corresponding to one of the second diode pairs, e.g. 722,724, discussed above. A dual diode structure 826, corresponding to oneof the first diode pairs, e.g. 714, 716, discussed above, includesanother N-type doped silicon region 828.

The guard layer 822 carries the −2.25V to the center n layer of the tieddual diode structure 824. As a result, as discussed above, thecapacitance between the trace 820 to the pickoff capacitor is referencedto this −2.25V, not to Vmc at the substrate 800.

It will be understood that the same structure applies for the other twodiode pairs for the other pickoff capacitor.

Each of the tied dual diode structures maybe advantageously formed bythe following process during manufacture of the sensor wafer. First, oneor more holes, depending on the desired size of the diodes, is cutthrough the sensor wafer, and then the holes are doped with phosphorousand treated with a heating step to create the n region of a p-n-pstructure. The phosphorous doping is also extended to form the extendedn-type region on the surface of the substrate. This is shown in FIG. 13,which is a schematic cross-sectional view of the substrate.

The surface is then oxidized to form an insulating layer (the bulksilicon oxide layer), and a contact for the motor common voltage traceis made through the oxide layer somewhere near the extended n-typeregion. Because the doping is extended to lie under the pickoff leadpath,

another wirebond pad is created at the existing row of pads to connectthe −2.25V to the n-type region. This structure creates a diode to thesubstrate and a small oxide capacitor to its lead. The output signal(CSCOM in FIG. 11) is now referenced to the −2.25V. This isolates thesignal from noise on the motor common voltage.

FIG. 14 illustrates a variant of the embodiment of FIG. 12, wherein thestructure is the same, moving from left to right in the drawing, untilthe first “break.” Here, instead of the bulk silicon oxide layercontinuing with area 802 d, as in FIG. 12, it continues with an area 802f extending without any via to a first dual diode structure 850. Thereis no underlying n-type guard layer, but rather each of the two dualdiode structures 850, 852 includes a respective n-type region 854, 856.The pad 858 receiving the −2.25V from ASIC sits on top of the area 802f, and a guard metal layer extension 860 extends from the pad 858.Another bulk silicon oxide area 802 g is formed with a via 862 betweenit and area 802 f. The guard metal extension 860 extends past the area802 f and down through the via 862 to the substrate 800, where itconnects with the n-type region 854 of the first diode structure 850.This ties the first diode structure 850 to the −2.25V reference.

To isolate the pickoff pad 864 from the guard metal extension 860, aninsulating layer 866 of, for example, sputtered glass is deposited onthe guard metal extension 860 so as to cover at least the via 862. Thepickoff pad 864 is formed on the insulating layer 866, with its trace868 leading off to the respective pickoff capacitor. As a result, as inthe embodiment of FIG. 12, the capacitance between the trace 866 to thepickoff capacitor is referenced to this −2.25V, not to the Vmc at thesubstrate 800.

While the embodiment of FIG. 14 provides good isolation in theory, thereare problems associated with manufacturing its structure that may makeit impractical. For example, an extra step is required to deposit thesputtered glass insulating layer 866, and the dimensions of the pickoffpad 864 have to be fairly small.

Therefore, in accordance with yet a further aspect of the presentinvention, the issue of capacitive coupling of voltage noise through thesubstrate is addressed not by isolating the pickoff capacitors from thesubstrate, but rather by eliminating this noise from the substrate inthe first place. More particularly, an advantageouscross-over/cross-under structure is proposed in which the substrateitself is grounded, resulting in no noise to shield out.

This structure is shown in the embodiment of FIG. 15. Here the siliconsubstrate 900 is selectively covered by a bulk oxide layer 902 that, inthe illustrated portion, includes only three areas 902 a, 902 b, 902 c.A via 903 between areas 902 a and 902 b permits ground voltage Vg onwire 904 from the LTCC board to be connected down to the substrate 900.On the other hand, the motor common voltage Vmc (here −45V) from theLTCC board is supplied to a pad 906 on area 902 b, which is connected toa trace 908 leading to the advantageous cross-over/cross-under structure910.

Moving upwardly from the oxide area 902 b, the structure 910 includes aconductive doped polysilicon layer 912 which functions as a cross-underlayer carrying the Vmc to the motor common. An insulating polysiliconoxide layer 914 completely covers the top and sides of the doped layer912 except at first and second vias 916, 918. The trace 908 carrying Vmcenters the via 916 that carries Vmc down to the doped layer 912.Metallization cross-over traces 920 for connections to the pickoffcapacitors are formed on the polysilicon oxide layer 914, and thereforeare isolated thereby from Vmc on the doped layer 912. The other via 918permits Vmc to come up from the doped layer 912 to a trace 924 leadingto the motors.

In this embodiment, there is only one diode structure 926, correspondingto one of the first diode pairs of FIG. 8 and hence not tied to the−2.25V reference. Indeed, because the substrate 900 is grounded, thereis no need for isolating the pickoff capacitor traces from the substrate900. Consequently, the diode structure 926 includes an N-type region 928below the bulk silicon oxide area 902 c, and the pickoff pad 930 and thetrace 932 therefrom are formed on top of the area 902 c.

The embodiment of FIG. 15 is highly advantageous in that it not onlyprovides a substantially noise-free output signal without voltage sourcenoise, but it also is readily manufacturable without the types ofconcerns identified for the previous embodiment. In particular, thedoped polysilicon layer 912 can be formed at the same time as thephysical capacitors using the same layer. No second double diodestructure tied to the front-end reference is required, and therefore novia thereto is required, although vias connecting the doped layer 914 tothe metal traces are added.

FIG. 16 is a top view of a CLA incorporating the advantageouscross-over/cross-under structure in accordance with the presentinvention. The doped layer 912 is circled for ease of reference, but itlies underneath the illustrated features. The motor common voltage Vmccomes up from the trace 908 at the bottom of FIG. 16 and travels upalong the lefthand side to via 916. Vmc then goes down to the dopedlayer 912 and travels under the motor teeth MT and pickoff teeth PT tocome up at via 918. From there, Vmc is fed to the two motors on thelefthand side without having generated noise in the effectively isolatedpickoff capacitors. The same structure is of course duplicated on therighthand side of FIG. 16 for the other two motors.

FIG. 17 is a circuit diagram of the embodiment of FIG. 15. As shown atthe left of the diagram, the substrate structure S2 is grounded and theeffective resistances are reduced. The upper and lower pickoff capacitorbranches B1, B2 each include only the single dual diode structure 926(0.25 pf) in series with the respective isolation capacitance Ciso (30pf).

Technical Sensor Design Advantages

An analog sensor with adequate rate sensitivity can meet the low randomwalk numbers over a 80 Hz bandwidth rather easily. This is because thenoise can be filtered with a quadratic low-pass filter at the signalsource. With an analog sensor, axis-alignment can be implemented bysumming in an offset to the capacitor pick-off plates. By using a sinedemodulated rate channel output, a high bandwidth servo loop cancontinuously null the major bias error source and compensate forthermal, mechanical package stress, initial processing and wafer maskerrors. Another big advantage will be to run the mechanism in a oneatmosphere pressure since this reduces the hermeticity requirement to areadily achievable level. This works well with the large package volumeof 2-3 cubic inches instead of trying to seal six separate sensors witha very tiny IC package. By using a magnetic drive, there is sufficientforce available to drive the mechanism at a one atmosphere ambient.

The silicon sensor design has evolved from an all KOH design which iswell know in the prior art, through several versions of DRIE through thewafer designs. The concept in accordance with the present invention is auniform projection through only the top 12% of the wafer thickness. Thissignificantly simplifies the design as there are no interactions betweenKOH process steps and DRIE steps. Also, feature smoothness is greatlyenhanced as the DRIE process is very good for cuts below 200 μm indepth. The design incorporates benefits such as symmetry, immunity toprocessing variations and servo rebalance to improve performance.

In accordance with yet another embodiment of the present invention, anadvantage in this sensor design is that the entire rate sensor andaccelerometer are defined by a single processing mask. This approachdiffers from typical surface micromachining in that the current sensorsuse a lightly doped silicon substrate to form the sensor mechanism. Thisgyro approach is inherently more stable than a polysilicon surfacemicromachined sensor or other surface micromachined sensor that uses aheavily doped silicon layer as its sensing element. This design willallow a mature, rapid process to cut just about any shape projected tothe desired thickness. Finite Element Analysis is more straightforwardsince the sensor is just the sensor geometry extruded into a uniformthickness. Models are easier to build and run, and they are a very exactapproximation to the actual silicon shape realized by DRIE. The siliconmask also becomes easier since it is just a single oxide mask withoutcorner compensation for complex three-D shapes. The result of the aboveadvantages is a greatly reduced design/fab/test cycle time. A siliconiteration can be done in a third to half of the time of bulk machineddesigns well known in the prior art. The hidden advantage with aprojection type sensor is the inherent “center flexure” design of havingthe z-axis CG (center of gravity) located precisely at the mid-point ofthe sensor layer thickness. A center flexure, which will greatly reduceunwanted modes, is difficult to achieve with prior art bulk silicondesigns.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the method and system of thepresent invention without departing from the spirit or scope of theinvention. Thus, it is intended that the present invention includemodifications and variations that are within the scope of the appendedclaims and their equivalents.

In addition, to the extent needed to understand any of the components orfeatures that are included in the present invention, the disclosures ofany and all of the patents mentioned herein are expressly incorporatedby reference thereto.

APPENDIX A: Reference to Related Patents

Reference is made to the following commonly assigned patents:

1) entitled “Control Circuit For Accelerometer,” U.S. Pat. No. 4,336,718issued on Jun. 29, 1982, in the name of John R. Washburn;

2) entitled “Monolithic Accelerometer,” U.S. Pat. No. 5,165,279 issuedon Nov. 24, 1992, in the name of Brian L. Norling;

3) entitled “Accelerometer With Co-Planar Push-Pull Force Transducers,”U.S. Pat. No. 5,005,413 issued on Apr. 9, 1991, in the name of MitchNovack;

4) entitled “Coriolis Inertial Rate and Acceleration Sensor,” U.S. Pat.No. 5,168,756 issued on Dec. 8, 1992, in the name of Rand H. Hulsing II;

5) entitled “Torque Coil Stress Isolator,” U.S. Pat. No. 5,111,694issued on May 12, 1992, in the name of Steven Foote;

6) entitled “Micromachined Rate And Acceleration Sensor,” U.S. Pat. No.5,627,314 issued on May 6, 1997, in the name of Rand H. Hulsing II;

7) entitled “Micromachined Rate And Acceleration Sensor,” U.S. Pat. No.5,557,046 issued on Sep. 17, 1996, in the name of Rand H. Hulsing II;

8) entitled “Micromachined Rate And Acceleration Sensor Having VibratingBeams,” U.S. Pat. No. 5,331,854 issued on Jul. 26, 1994, in the name ofRand H. Hulsing II;

9) entitled “Micromachined Rate And Acceleration Sensor,” U.S. Pat. No.5,241,861 issued on Sep. 7, 1993, in the name of Rand H. Hulsing II;

10) entitled “Capacitance Type Accelerometer For Air Bag System,” U.S.Pat. No. 5,350,189 issued on Sep. 27, 1994, in the name of ShiegekiTsuchitani et al.;

11) entitled “Differential Capacitive Transducer And Method Of Making,”U.S. Pat. No. 4,825,335 issued on Apr. 25, 1989, in the name of LeslieB. Wilner;

12) entitled “Miniature Silicon Accelerometer And Method,” U.S. Pat. No.5,205,171 issued on Apr. 27, 1993, in the name of Benedict B. O'Brian etal.;

13) entitled “Low Vibration Link,” U.S. Pat. No. 6,098,462, issued onAug. 8, 2000, in the name of Rand H. Hulsing II;

14) entitled “Low Vibration Link,” U.S. Pat. No. 6,079,271, issued onJun. 27, 2000, in the name of Rand H. Hulsing II;

15) entitled “Axis Alignment Method,” U.S. Pat. No. 6,276,203, issued onAug. 21, 2001, in the name of Rand H. Hulsing, II; and

16) entitled “Micromachined Rate and Acceleration Sensor,” U.S. Pat. No.5,319,976, issued on Jun. 14, 1994, in the name of Rand H. Hulsing, II.

1. An apparatus for measuring the angular rotation of a moving body, theapparatus comprising: an upper sensor layer; a lower handle layersubstantially parallel to the sensor layer; at least one dither frameformed of the upper sensor layer, the frame having a dither axisdisposed substantially parallel to the upper sensor layer and the lowerhandle layer; a first accelerometer formed of the upper sensor layer andhaving a first force sensing axis perpendicular to the dither axis forproducing a first output signal indicative of the acceleration of themoving body along the first force sensing axis, the first accelerometerhaving a proof mass and at least one flexure connecting the proof massto the dither frame such that the proof mass can be electrically rotatedperpendicular to the dither axis; a second accelerometer formed of theupper sensor layer and having a second force sensing axis perpendicularto the dither axis for producing a second output signal indicative ofthe acceleration of the moving body along the second force sensing axis,the second accelerometer having a proof mass and at least one flexureconnecting the proof mass to the dither frame such that the proof masscan be electrically rotated perpendicular to the dither axis; the ditherframe and proof masses having electrodes on an insulating layer foroperating the first and second accelerometers; and the upper sensorlayer having a rate axis perpendicular to each of the first and secondforce sensing axes and the dither axis, whereby the first and secondoutput signals have a Coriolis component indicative of the angularrotation of the moving body about the rate axis.
 2. The apparatus ofclaim 1, wherein the lower handle layer is bonded to the sensor layer atthe insulating layer.
 3. The apparatus of claim 1, wherein theinsulating layer is an oxide layer.
 4. The apparatus of claim 1, whereinthe insulating layer acts as an etch stop for an etching method.
 5. Theapparatus of claim 4, wherein the etching method is a potassiumhydroxide or tetramethyl ammonium hydroxide method.
 6. The apparatus ofclaim 1, wherein the proof masses of the first and second accelerometersare of H shape.
 7. The apparatus of claim 1, wherein the first andsecond accelerometers are connected by a link.
 8. The apparatus of claim1, wherein the dither frame is driven magnetically, while the first andsecond accelerometers are driven capacitively.
 9. The apparatus of claim1, wherein the first and second accelerometers are translational masses.10. The apparatus of claim 1, further comprising first and secondcapacitor plates positioned perpendicular to and spaced between theupper sensor layer and the lower handle layer.
 11. The apparatus ofclaim 10, wherein each of the first and second capacitor plates comprisethe electrodes.
 12. The apparatus of claim 11, wherein the first andsecond accelerometers comprise capacitive force rebalanceaccelerometers.
 13. The apparatus of claim 12, further comprising aforce-rebalance circuit coupled to the first and second accelerometersfor balancing applied acceleration forces to restore each of the proofmasses to neutral position between the first and second capacitorplates.
 14. The apparatus of claim 13, wherein the first and secondforce rebalancing, accelerometers include an electronic bias voltage forcancellation of mechanical forces.
 15. The apparatus of claim 1, whereinthe apparatus is operated at one atmosphere pressure and a predetermineddither frequency is nominally 8kHz.
 16. The apparatus of claim 1,wherein the pair of flexures each comprise at least one dither leg. 17.The apparatus of claim 1, further comprising a magnetic circuitgenerating a magnetic flux that intersects the at least one ditherframe.
 18. The apparatus of claim 17, further comprising a conductivepath deposited on at least one of the flexures.
 19. The apparatus ofclaim 18, further comprising an electrical circuit coupled to theconductive path and generating a drive signal therein, the drive signalinteracting with the magnetic flux to impart a dithering motion to eachof the first and second accelerometer frames having a predeterminedfrequency along the dither axis to generate the first and second outputsignals.
 20. The apparatus of claim 19, wherein the electrical circuitincludes a signal processor for processing a pick-off signal and furthercomprising: a second conductive path disposed on the upper sensor layerto traverse the first and second accelerometer frames, the secondconductive path coupled to the electrical circuit and intersected by amagnetic flux which generates a pick-off signal in the second conductivepath representative of the dithering of the first and secondaccelerometer frames along the dither axis.
 21. The apparatus of claim19, wherein the predetermined dither frequency and nominal accelerometerfrequency are set equal.
 22. A method of improving an accelerometer biasperformance, the accelerometer having a force rebalance circuit coupledthereto for restoring a proof mass to its neutral position betweencapacitor plates, the method comprising the steps of: adding a biasvoltage across capacitor gaps by coupling a power supply to a sensorsubstrate; and applying an electrical offset to the motor terminals,thereby resulting in an attractive force having a negative spring rateand effectively decreasing proof mass natural frequency.
 23. A method ofmeasuring the angular rotation of a moving body, the method comprisingthe steps of: forming an upper sensor layer; forming a lower handlelayer substantially parallel to the sensor layer; forming at least onedither frame of the upper sensor layer, the frame having a dither axisdisposed substantially parallel to the upper sensor layer and the lowerhandle layer; forming a first accelerometer of the upper sensor layerand having a first force sensing axis perpendicular to the dither axisfor producing a first output signal indicative of the acceleration ofthe moving body along the first force sensing axis, the firstaccelerometer having a proof mass and at least one flexure connectingthe proof mass to the dither frame such that the proof mass can beelectrically rotated perpendicular to the dither axis; forming a secondaccelerometer of the upper sensor layer and having a second forcesensing axis perpendicular to the dither axis for producing a secondoutput signal indicative of the acceleration of the moving body alongthe second force sensing axis, the second accelerometer having a proofmass and at least one flexure connecting the proof mass to the ditherframe such that the proof mass can be electrically rotated perpendicularto the dither axis; the dither frame and proof masses having electrodeson an insulating layer for operating the first and secondaccelerometers; and imparting a dithering motion to each of the firstand second accelerometers of a predetermined frequency along the ditheraxis, whereby the first and second output signals have a Corioliscomponent indicative of the angular rotation of the moving body aboutthe rate axis.
 24. The method of claim 23, further comprising the stepof bonding the lower handle layer to the sensor layer at the insulatinglayer.
 25. The method of claim 23, wherein the insulating layer is anoxide layer.
 26. The method of claim 23, wherein the insulating layeracts as an etch stop for an etching method.
 27. The method of claim 26,wherein the etching method is a potassium hydroxide or TMAH method. 28.The method of claim 23, wherein the proof masses of the first and secondaccelerometers are of H shape.
 29. The method of claim 23, furthercomprising the step of forming a link connecting the first and secondaccelerometers to maintain same dither frequency between each of thefirst and second accelerometers.
 30. The method of claim 23, furthercomprising the steps of driving the dither frame magnetically, anddriving the first and second accelerometers capacitively.
 31. The methodof claim 23, wherein the first and second accelerometers aretranslational masses.
 32. The method of claim 23, further comprising thestep positioning the first and second capacitor plates perpendicular toand spaced between the upper sensor layer and the lower handle layer.33. The method of claim 32, wherein each of the first and secondcapacitor plates comprise the electrodes.
 34. The method of claim 33,wherein the first and second accelerometers comprise capacitive forcerebalance accelerometers.
 35. The method of claim 34, further comprisingthe step of coupling a force-rebalance circuit to the first and secondaccelerometers for balancing applied acceleration forces to restore eachof the proof masses to neutral position between the first and secondcapacitor plates.
 36. The method of claim 35, wherein the first andsecond force rebalancing accelerometers include an electronic biasvoltage for cancellation of mechanical forces.
 37. The method of claim23, further comprising the step of operating the apparatus at oneatmosphere pressure and dithering the first and second accelerometers ata predetermined dither frequency of nominally 8 kHz.
 38. The method ofclaim 23, wherein the pair of flexures each comprise at least one ditherleg.
 39. The method of claim 23, further comprising the step ofgenerating a magnetic flux with a magnetic circuit, the magnetic circuitdisposed such that the magnetic flux intersects the at least one ditherframe.
 40. The method of claim 23, further comprising the step ofdepositing a conductive path on at least one of each of the flexures.41. The method of claim 40, further comprising the step of coupling anelectrical circuit to the conductive path and generating a drive signaltherein, the drive signal interacting with the magnetic flux to impart adithering motion to each of the first and second accelerometer frameshaving a predetermined frequency along the dither axis, whereby thefirst and second output signals have a Coriolis component indicative ofthe angular rotation of the moving body about the dither axis.
 42. Themethod of claim 41, wherein the electrical circuit includes a signalprocessor for processing a pick-off signal and further comprising thestep of: disposing a second conductive path on the upper sensor layer totraverse the first and second accelerometer frames, the secondconductive path coupled to the electrical circuit and intersected by amagnetic flux, whereby the magnetic flux generates a pick-off signal inthe second conductive path representative of the dithering of the firstand second accelerometer frames along the dither axis.
 43. The method ofclaim 41, wherein the predetermined dither frequency and nominalaccelerometer frequency are set equal.
 44. An apparatus for measuringthe angular rotation of a moving body, the apparatus comprising: anupper sensor layer; a lower handle layer substantially parallel to saidsensor layer; a capacitive pickoff structure for providing an outputsignal indicative of the angular rotation of the moving body, saidpickoff structure including first and second air capacitive elementsreceiving first and second measurement signals from which said outputsignal is derived; a first accelerometer motor formed of said sensorlayer, said first accelerometer motor including a first conductive pathfor providing said first measurement signal to said first capacitiveelement and a second conductive path for receiving a common motorvoltage, said common motor voltage including noise; a secondaccelerometer motor formed of said sensor layer, said secondaccelerometer motor including a third conductive path for providing saidsecond measurement signal to said second capacitive element and a fourthconductive path for receiving said common motor voltage; and anisolation structure for preventing substantial capacitive coupling ofsaid first and third paths to any other element of said apparatuscarrying said common motor voltage that would import the noise into saidfirst and second measurement signals.
 45. The apparatus of claim 44,wherein said handle layer is a silicon substrate and said sensor layeroverlies said substrate.
 46. The apparatus of claim 45, wherein: saidsubstrate carries said motor common voltage, said second path connectssaid substrate to said first accelerometer motor to provide said motorcommon voltage to said first accelerometer motor, said fourth pathconnects said substrate to said second accelerometer motor to providesaid motor common voltage to said second accelerometer motor, saidisolation structure includes at least one dual diode structure in saidsubstrate including a first dual diode structure for reducing capacitivecoupling between said first path and said substrate, and said isolationstructure further includes at least one further dual diode structure insaid substrate including a second dual diode structure for reducingcapacitive coupling between said third path and said substrate.
 47. Theapparatus of claim 46, wherein: said first dual diode structure is inseries with said first path and is tied to a reference voltage otherthan said motor common voltage, and said second dual diode structure isin series with said third path and is tied to a reference voltage otherthan said motor common voltage.
 48. The apparatus of claim 47, whereinsaid first dual diode structure is a p-n-p structure with a first nregion tied to said reference voltage, and said second dual diodestructure is a p-n-p structure with a second n region tied to saidreference voltage.
 49. The apparatus of claim 48, wherein: saidsubstrate is a p-type silicon substrate, said first n region extendsupwardly through said substrate to a first position on an upper surfaceof said substrate and along said upper surface to a second positionthereon, said isolation structure further includes a first bulk siliconoxide layer area on said upper surface of said substrate so as to coversaid first position and to leave said second position uncovered at afirst via, a first conducting pad on said first area to which said firstpath is connected, and a first tie connected to said first n regionthrough said first via to provide said reference voltage thereto, saidsecond n region extends upwardly through said substrate to a thirdposition on said upper surface of said substrate and along said uppersurface to a fourth position thereon, and said isolation structurefurther includes a second bulk silicon oxide layer area on said uppersurface of said substrate so as to cover said third position and toleave said fourth position uncovered at a second via, a secondconducting pad on said third area to which said third path is connected,and a second tie connected to said second n region through said secondvia to provide said reference voltage thereto.
 50. The apparatus ofclaim 48, wherein: said substrate is a P-type silicon substrate, saidfirst n region extends upwardly through said substrate to a firstposition on an upper surface of said substrate, said isolation structurefurther includes a first bulk silicon oxide layer area on said uppersurface of said substrate so as to leave said first position uncoveredat a first via, a first guard metal layer portion on said first area andextending down through said first via to connect to said first n region,a first conducting pad on said first guard metal layer portion connectedto a first tie to provide said reference voltage to said first n region,a first insulating layer portion on said first guard metal layerportion, and a second conducting pad on said first insulating layerportion to which said first path is connected, said second n regionextends upwardly through said substrate to a second position on saidupper surface of said substrate, and said isolation structure furtherincludes a second bulk silicon oxide layer area on said upper surface ofsaid substrate so as to leave said second position uncovered at a secondvia, a second guard metal layer portion on said second area andextending down through said second via to connect to said second nregion, a third conducting pad on said second guard metal layer portionconnected to a second tie to provide said reference voltage to saidsecond n region, a second insulating layer portion on said second guardmetal layer portion, and a fourth conducting pad on said firstinsulating layer portion to which said third path is connected.
 51. Theapparatus of claim 50, wherein said first and second insulating layerportions are formed of sputtered glass.
 52. The apparatus of claim 45,wherein: said substrate is grounded and said isolation structureincludes: a bulk silicon oxide layer area on an upper surface of saidsubstrate, a first conducting pad on said area being connected toreceive said motor common voltage from a voltage source; a conductivecross-under layer covering a first portion of said area; an insulatinglayer covering said cross-under layer, said insulating layer having atleast first and second vias therethrough; a first conductive traceconnected to said first pad and extending therefrom over said insulatinglayer and down through said first via to be connected to saidcross-under layer and provide said common motor voltage to saidcross-under layer; a second conductive trace connected to saidcross-under layer to receive said common motor voltage and extending upthrough said second via, said second conductive trace being connected tosaid second and fourth paths to provide said common motor voltagethereto; at least one cross-over conductive trace on said insulatinglayer and insulated thereby from said common motor voltage on saidcross-under layer; a second conducting pad on said bulk silicon oxidelayer at a position spaced from said cross-under layer, said first pathbeing connected to said second conducting pad; and a third conductingpad on said bulk silicon oxide layer at a position spaced from saidcross-under layer, said third path being connected to said thirdconducting pad.
 53. The apparatus of claim 52, wherein said cross-underlayer is formed of doped polysilicon.
 54. The apparatus of claim 52,wherein said insulating layer is formed of polysilicon oxide.
 55. Theapparatus of claim 52, wherein said isolation structure includes a firstdual diode structure in said substrate for reducing capacitive couplingbetween said first path and said substrate, said first dual diodestructure being in series with said first path, and said isolationstructure further includes a second dual diode structure in saidsubstrate for reducing capacitive coupling between said third path andsaid substrate, said second dual diode structure being in series withsaid third path.
 56. The apparatus of claim 45, wherein: said substrateis grounded, a bulk silicon oxide layer area is provided on an uppersurface of said substrate, and a first conducting pad is provided onsaid area connected to receive said motor common voltage from a voltagesource, wherein said isolation structure is a cross-under/cross-overstructure including a cross-under layer over said bulk silicon oxidelayer area, an insulating layer over said cross-under layer andcross-over traces over said insulating layer, wherein said firstconducting pad is connected to said cross-under layer to provide saidmotor common voltage thereto, and said cross-over traces are insulatedfrom said cross-under layer by said insulating layer.
 57. The apparatusof claim 45, wherein said substrate is grounded, and said isolationstructure is a cross-under/cross-over structure insulated from saidsubstrate and including a layer carrying said common motor voltage, atleast one trace and an insulating layer insulating said trace from saidfirst layer